dts-binding: modem: wnc-14a2a: remove base_label to fix build errors

When the WNC-14A2A modem binding was originally introduced, I thought
the base_label would shorten the define keeping the result short and
easily portable.  Turns out with the latest changes, it has a side
effect of removing the "DT_*" prefix which is breaking the build.

Let's remove "base_label" from the modem binding and adjust all of
the dts_fixups referring to the WNC14A2A defines.

NOTE: This commit moves the left-over WNC14A2A dts_fixup defines from
the nRF52 soc into samples/net/lwm2m_client as the new values.
They will stay there until the modem can be re-configured as a shield.

Signed-off-by: Michael Scott <mike@foundries.io>